pandas.DataFrame.unstack

官方文档：

pandas.DataFrame.unstack¶

DataFrame. unstack ( level=-1, fill_value=None ) [source] ¶

Pivot a level of the (necessarily hierarchical) index labels, returning a DataFrame having a new level of column labels whose inner-most level consists of the pivoted index labels. If the index is not a MultiIndex, the output will be a Series (the analogue of stack when the columns are not a MultiIndex).（当行索引只有一个时，结果会生成一个Series，详解见下面样例） The level involved will automatically get sorted.

Parameters:	level : int, string, or list of these, default -1 (last level) Level(s) of index to unstack, can pass level name fill_value : replace NaN with this value if the unstack produces missing values
Returns:	unstacked : DataFrame or Series

See also

DataFrame.pivot

Pivot a table based on column values.

DataFrame.stack

Pivot a level of the column labels (inverse operation from unstack).

Examples

>>> index = pd.MultiIndex.from_tuples([('one', 'a'), ('one', 'b'),
...                                    ('two', 'a'), ('two', 'b')])
>>> s = pd.Series(np.arange(1.0, 5.0), index=index)
>>> s
one  a   1.0
     b   2.0
two  a   3.0
     b   4.0
dtype: float64

>>> s.unstack(level=-1)
     a   b
one  1.0  2.0
two  3.0  4.0

>>> s.unstack(level=0)
   one  two
a  1.0   3.0
b  2.0   4.0

>>> df = s.unstack(level=0)
>>> df.unstack()
one  a  1.0
     b  2.0
two  a  3.0
     b  4.0
dtype: float64

这里返回的Series本来是：

One     Two
a  b       a  b
1  2       3  4

这样一个dateframe，同样是将行索引转化为列索引，只不过这样数据就只有一行了，所以自动转化为Series.